Method of manufacturing surface acoustic wave device

ABSTRACT

Disclosed is a method of manufacturing a surface acoustic wave device comprising the steps of forming a drive electrode having a surface acoustic wave element function on a piezoelectric substrate wafer, providing a resist coat on an upper region of the drive electrode, covering the resist coat with a metal film, removing the resist coat lying within the metal film so that the metal film is formed in a dome form having a hollow portion covering the drive electrode, and providing a resin seal on the metal film.

This is a divisional of application Ser. No. 09/931,006, filed Aug. 17,2001, now U.S. Pat. No. 6,573,635.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a surface acoustic wave (SAW)device, and, more particularly, to a surface acoustic wave deviceenabling packaging to be effected in a wafer process.

2. Description of the Related Art

Mobile communication equipment such as cellular phones and cordlesstelephones is rapidly prevailing with recent progress towardminiaturization and lightness of electronic devices. A high frequencycircuit of a radio communication circuit included in such mobilecommunication equipment uses a multiplicity of electronic devicesmounted with filter elements.

For the purpose of realizing the miniaturization and lightness inparticular, surface acoustic wave (SAW) elements are employed as thefilter elements. FIG. 1 is a schematic diagram showing in section aconventional surface acoustic wave device having a surface acoustic waveelement, and its wiring connection structure for connecting the surfaceacoustic wave element to external connection terminals.

A package for the surface acoustic wave device is constructed from amultilayer ceramic package 100 and a metal cap 101. The surface acousticwave element 104 is adhered by an electroconductive resin 105 to the topof a substrate 103 disposed within the interior of the package, withinput and output terminals of the surface acoustic wave element 104being electrically connected via aluminum wires 106 to a groundterminal. The reverse side of the substrate 103 is formed with anexternal connection terminal 107.

FIG. 2 shows the structure of another conventional surface acoustic wavedevice, in which the surface acoustic wave element 104 is connected byconnection bumps 108 to the substrate 103 disposed on the bottom of thepackage, to provide physical fixation and electrical connection wiring.

Thus, in the structure shown in FIGS. 1 and 2, electric wirings (thealuminum wires 106 in FIG. 1 and the connection bumps 108 in FIG. 2) areboth formed within the interior of the ceramic package 100.

The cap 101 has a sealing material 109 formed in a region in contactwith the ceramic package 100. This provides a hermetic sealing betweenthe ceramic package 100 and the cap 101 so that airtightness is heldwithin the interior of the package.

Thus, to achieve a miniaturization of the surface acoustic wave device,the structure shown in FIGS. 1 and 2 can not neglect the space which isused for the aluminum wire connection and the hermetic sealing structurebetween the package 100 and the cap 101.

The manufacturing procedure includes making electrode wiring on apiezoelectric substrate wafer by patterning and thereafter cutting andseparating the wafer into chip elements to thereby obtain individualsurface acoustic wave (SAW) elements 104.

The cut chip elements are mounted on the package 100, which is thenfitted with the cap 101 for sealing to obtain a surface acoustic wavedevice. For this reason, the cost of the cap 101 is a factor greatlyaffecting the price of product of the surface acoustic wave device. Onthe contrary, another technique is also known where the package isformed in the state of a wafer (Japan Patent Laid-open Pub. No.2000-261285).

In the technique described in Japan Patent Laid-open Pub. No.2000-261285, electrodes are formed on a piezoelectric substrate wafer bypatterning and a cover forming member is formed from a separate andindependent substrate wafer. The cover forming member is then laminatedto the piezoelectric substrate wafer having the electrodes formedthereon by patterning, to thereby obtain a surface acoustic wave devicehaving a surface acoustic wave element function.

However, such a technique disclosed in the above patent laid-openpublication also imposes a limitation on miniaturization of the surfaceacoustic wave device and needs a separate provision of the cover formingmember, which may be disadvantageous in the number of manufacturingsteps. This leads to increase the price of the device.

SUMMARY OF THE INVENTION

The present invention was conceived in view of the problems involved inthe prior art. It is therefore an object of the present invention toprovide a lightweight and chip-sized surface acoustic wave device.

It is another object to provide a surface acoustic wave device capableof being manufactured up to packaging in the state of a piezoelectricsubstrate wafer by a less number of steps.

In order to achieve the above objects, according to a first aspect ofthe present invention there is provided a surface acoustic wave devicecomprising a piezoelectric substrate; a drive electrode unit formed onthe piezoelectric substrate, for generating surface acoustic waves; andan electrically conductive electrode protecting unit for covering thedrive electrode unit with a hollow therebetween, wherein the electrodeprotecting unit is formed on the piezoelectric substrate by use of afilm forming technique.

In order to achieve the above objects, according to a second aspect ofthe present invention there is provided a surface acoustic wave devicecomprising a piezoelectric substrate; an electrode unit formed on thepiezoelectric substrate, the electrode unit including a drive electrodeunit for generating surface acoustic waves and an external connectionelectrode unit; an electrically conductive electrode protecting unit forcovering the drive electrode unit with a hollow therebetween, theelectrode protecting unit being formed on the piezoelectric substrate byuse of a film forming technique; an electroconductive column formed onthe external connection electrode unit; and an external connectionterminal formed at the extremity of the electroconductive column,wherein the piezoelectric substrate is sealed by a resin with theexception of the external connection terminal and the electrodeprotecting unit.

In order to achieve the above objects, according to a third aspect ofthe present invention there is provided a method of manufacturing asurface acoustic wave device, comprising the steps of forming a driveelectrode having a surface acoustic wave element function on apiezoelectric substrate wafer; resist coating an upper region of thedrive electrode; effecting a metal film coating in dome form so as tocover the resist coat; removing the resist lying within the metal domecoated; and providing a resin seal thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, aspects, features and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiment when read in conjunction withthe accompanying drawings, in which:

FIG. 1 is a schematic view showing in section a conventional surfaceacoustic wave device having a surface acoustic wave element, and itswiring connection structure for connecting the surface acoustic waveelement to an external connection terminal;

FIG. 2 is a diagram showing the configuration of another conventionalsurface acoustic wave device, depicting the features of physicalfixation by flip-chip bonding and of electrical connection wiring;

FIG. 3 is a sectional view showing the schematic structure of a surfaceacoustic wave device having a surface acoustic wave (SAW) elementfunction in accordance with the present invention;

FIG. 4 is a diagram showing the details of a connection terminalstructure unit for signal path of FIG. 3;

FIG. 5 is a top plan view of an embodiment of the surface acoustic wavedevice having a surface acoustic wave (SAW) element filter formedthereon in accordance with the present invention;

FIG. 6 is a sectional view of the surface acoustic wave element whenviewed from below in FIG. 5;

FIG. 7 shows the structure of a detailed example of an externalconnection electrode pad 200 of FIG. 6;

FIGS. 8A to 8C are diagrams explaining the details of a metal dome 3;

FIGS. 9A to 9H are diagrams showing the step of forming the surfaceacoustic wave device; and

FIG. 10 is a diagram showing the detailed structure of a processing step(FIG. 9H) for a resin seal 41, in addition to the metal dome 3 and ametal column 4 formed in the step of FIGS. 8A to 8C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a sectional view showing the schematic structure of a surfaceacoustic wave device having a surface acoustic wave (SAW) elementfunction in accordance with the present invention.

Referring to FIG. 3, an electrode unit 2 formed on a piezoelectricsubstrate 1 comprises external connection electrode units 20electrically connected to external connection terminals 5 for signalpath, and a drive electrode unit 21 having an excitation and reflectionelectrode acting as a surface acoustic wave (SAW) element function unit.

As one feature, in order to hold the airtightness, a dome-shapedelectrode protecting unit 3 provides a hollowness for the top surface ofthe drive electrode unit 21 having the excitation and reflectionelectrode acting as the surface acoustic wave (SAW) element functionunit.

The external connection electrode unit 20 acts as an electrode pad andhas, on its top surface, a metal column 4 connected to the externalconnection terminals 5. The end surface of each metal column 4 is formedwith the external connection terminal 5 made of a metal bonding materialsuch as lead free solder. Furthermore, with the exception of the surfaceof the electrode protecting unit 3 and the portion of the externalconnection terminal 5, a resin seal 6 is provided.

By virtue of the electrode protecting unit 3, it is possible to hold theairtightness above the drive electrode unit 21 for excitation andreflection to thereby secure the reliability of the surface acousticwave device.

Such a characteristic structure of the present invention eliminates theneed for the cap and packaging and allows the surface acoustic wavedevice to be formed in the state of a piezoelectric substrate wafer.

FIG. 4 shows the details of a connection terminal structure unit forsignal path of FIG. 3. An electrically conductive layer 22 is formed ontop of the external connection electrode unit 20 to increase the bondingstrength between the external connection electrode unit 20 and the metalcolumn 4.

Furthermore, due to the provision of an electrically conductiveintermediate layer 40 between the end surface of the metal column 4 andthe end surface of the external connection terminal 5 made of a metalbonding material, it is possible to prevent the diffusion of componentsof the metal column 4 into the external connection terminal 5 made ofthe metal bonding material. These contribute to enhancement of thereliability of the surface acoustic wave device.

FIGS. 5 and 6 show an embodiment of the present invention. FIG. 5 is atop plan view of the embodiment of a surface acoustic wave device havinga surface acoustic wave (SAW) element filter in accordance with thepresent invention. FIG. 6 is a sectional view of the surface acousticwave device when viewed from below of FIG. 5.

A LiTaO₃ single crystal substrate (hereinafter referred to as an LTsubstrate 1) is used as the piezoelectric substrate 1 of the surfaceacoustic wave device shown in FIGS. 5 and 6. Besides the LiTaO₃ singlecrystal substrate, substrate materials such as LiNbO₃ single crystal andquartz having a piezoelectric effect are available for the piezoelectricsubstrate.

Drive electrode units 21, signal line connection electrode pads 200 andground connection electrode pads 201 as the external connectionelectrode units 20 are formed on the LT substrate 1, each electrode padbeing made mainly of Al. Al wiring patterns 202 connect each electrodepad of the signal line connection electrode pads 200 and the groundconnection electrode pads 201, and corresponding one of the driveelectrode units 21.

To obtain appropriate filter characteristics, a hollow space needs to beformed above the drive electrode unit 21 for excitation and reflection.For this reason, in the present invention, a metal dome 3 is formed onthe drive electrode unit 21 to secure a hollow space 210. In theembodiment, the metal dome 3 is formed by use of a film formingtechnique such as plating, sputtering or vapor deposition with Cu orwith a material mainly containing Cu.

The metal dome 3 is connected to any ground wiring pattern so that theentire metal dome is at the ground potential.

FIG. 7 shows the structure of a detailed example of the externalconnection electrode pad 200 of FIG. 6. The ground electrode pad 201being similar in structure to the input/output electrode pad 200, onlythe input/output electrode pad 200 is visible in FIG. 7.

An insulating intermediate layer 10 formed on the LT substrate 1 in FIG.4 is formed from an SiO₂ insulating film.

The signal line connection electrode pads 200 and the ground connectionelectrode pads 201 are made of an Al-containing metal by patterning theSiO₂ insulating film acting as the intermediate layer 10. A Ti film isfurther formed as an electrically conductive layer 22 on top of theAl-containing signal line connection electrode pads 200 by vapordeposition or sputtering.

The metal column 4 made mainly of Cu is formed on top of theelectrically conductive intermediate layer 22. The Ti film layer of theelectrically conductive intermediate layer 22 contributes to anenhancement of the intimate adhesion between the signal line connectionelectrode pad 200 and the metal column 4.

After the formation of the metal column 4, an Sn film is formed as theelectrically conductive intermediate layer 40 on the end surface of themetal column 4 by plating, vapor deposition, sputtering or the like. TheSn film prevents Cu component of the metal column 4 from diffusing intothe external connection terminal 5 formed from lead free solder as ametal bonding material.

In the embodiment shown in FIG. 6, the surface of the metal dome 3 isalso formed with the external connection terminal 5 made of lead freesolder as a metal bonding material. Accordingly, an Sn film may beformed as the electrically conductive intermediate layer 40 on top ofthe metal dome 3.

With the exception of the external connection terminals 5 made of thelead free solder as the metal bonding material and the metal domes 3,the surface of the LT substrate 1 is coated with epoxy resin, polyimideresin or the like for hardening.

The details of the metal dome 3 will then be described with reference toFIGS. 8A to 8C as well as FIGS. 9A to 9H which illustrate formationsteps of the surface acoustic wave device.

FIG. 8A is a schematic diagram showing the arrangement pattern of themetal columns 4 and the position of formation of the metal dome 3 whenperspectively viewing from top of the surface acoustic wave devicehaving the surface acoustic wave (SAW) element function.

Description will now be made with reference to FIGS. 9A to 9H. A resistA is coated on the LT substrate 1 which is a piezoelectric substratewafer (processing step FIG. 9A). FIGS. 9A to 9H show only a singlesurface acoustic wave device portion cut from the piezoelectricsubstrate wafer on a chip-to-chip basis.

Although previous to the processing step FIG. 9A, the drive electrodeunit 21 making up the surface acoustic wave element function unit andthe external connection electrode unit 20 connected to the externalconnection terminal 5 are formed by patterning on the LT substrate 1,these electrode units are regarded as being already formed and the stepstherefor are not shown.

Patterning is then made in such a manner as to leave the regioncorresponding to the hollow space 210 of the metal dome 3, and theresist A is removed (processing step FIG. 9B).

A resist B is then coated to a height which is level with the topsurface of the metal dome 3 (processing step FIG. 9C). Thus, in terms ofthe resist films formed as shown in FIG. 9C, the resist A filmcorresponding to the hollow forming portion 210 (see FIG. 6) of themetal dome 3 is formed so as to be thinner than the resist B film formedaround the metal dome 3.

Afterward, the resist B lying within the regions corresponding to themetal column 4 and the metal dome 3 is removed by patterning (processingstep FIG. 9D).

FIG. 8B is a sectional view taken along a center line k of FIG. 8A. FIG.8B, a schematic view corresponding to FIGS. 9A to 9H, shows the resist Aand B films which are formed with the exception of the region where themetal dome 3 is formed. In FIG. 8B, the patterning of the metal column 4portion is not shown.

Then, with the resist A film and the resist B film formed, a Cu film isformed to a thickness h of 100 μm by electrolytic plating to form themetal column 4 and the metal dome 3 (processing step FIG. 9E). Acurrent-feed terminal for electrolytic plating is not shown. Electrolessplating may be effected.

The resist B film is then removed and the resist A film corresponding tothe hollow space 210 covered by the metal dome 3 is removed from anoutlet for resist 30 (processing step FIG. 9H). This allows the hollowspace 210 to be formed within the interior of the metal dome 3 as shownin FIG. 8C.

The order of removal of the resist B film around the metal dome and theresist A film within the hollow space 210 is as follows. The resist Bfilm is first removed, and then the resist A film is removed from theoutlet for resist 30 by using a solvent. The removal of the resist Afilm and B film may be effective in a consecutive manner.

In FIGS. 8A to 8C, the outlet for resist 30 is disposed at the top andbottom (in FIG. 8A),which is shown in a simplified manner for thepurpose of understanding.

More specifically, the outlet for resist 30 provided in the metal dome 3portion is arranged such that it lies on the input/output signal line(the aluminum wiring pattern 202 of FIG. 5) connecting the electrodepattern of the metal column 4 and the drive electrode 21 covered by themetal dome 3. This obviates any contact of the metal dome 3 with theinput/output signal line. Accordingly, in the example of FIG. 5, theoutlet for resist 30 is provided at three locations of the top and ofthe bottom corresponding to the aluminum wiring pattern.

Returning to FIGS. 9A to 9H, lead free solder (Sn—Ag—Cu) is fused to theextremity of the metal column 4 formed simultaneously with the metaldome 3, to form the external connection terminal 5 (processing step FIG.9G). With the exception of the external connection terminal 5 and themetal dome 3, the surface is formed with the resin sealing (processingstep FIG. 9H).

As a result, an electronic device having a surface acoustic wave (SAW)element formed thereon is obtained.

Although FIGS. 9A to 9H show the manufacturing steps of the surfaceacoustic wave device in the form of a single chip for the purpose ofsimplification as described above, a plurality of chips are typicallyformed on the LT substrate 1 like the piezoelectric substrate wafer, andat the final stage of the process it is cut and separated into chips toobtain individual surface acoustic wave devices.

FIG. 10 is a diagram further showing the detailed structure of theprocessing step of the resin sealing 41 on the metal dome 3 in FIG. 9Hand the metal column 4 formed by the steps shown in FIGS. 8A to 8C.

In the processing step of the resin sealing 41 in FIG. 9H, a layer ofepoxy b42 is formed on top of the LT substrate 1 to seal off the outletfor resist 30. The layer of epoxy b42 covers the input/output signalline (aluminum wiring pattern 202) and enters the space defined betweenthe line and metal dome 3 to improve the electrical insulatingproperties between the input/output signal line and the metal dome 3.

After hardening of the epoxy b42 layer, a layer of epoxy a41 is coatedthereon up to a height which is level with the Cu metal column 4 and themetal dome 3, for hardening.

The conditions to be fulfilled between the sealing resin layers of theepoxy resin a41 and epoxy resin b42 is that the sealing resin layer ofthe epoxy b42 providing a lower layer has a shorter hardening time andhigher viscosity than the sealing resin layer of the epoxy a41 providingan upper layer.

This is attributable to the need for the epoxy b42 to cover the outletfor resist 30 portion of the metal dome 3 but not to enter the interiorof the hollow space upon the formation of the epoxy b42. For thisreason, the epoxy resin b42 requires hardening in a shorter period oftime and a high viscosity. It is also desirable upon the formation ofthe layer of the epoxy a41 to form an even layer. Thus, the aboveconditions are imposed to the relationship of the hardening speed and ofthe viscosity between the epoxy a41 and the epoxy b42.

The above layer of the epoxy b42 may directly be formed in the region ofthe outlet for resist 30 by only potting without being necessarilyformed as a resin layer having an evenness.

The epoxy a41 and b42 may be replaced by other resin such as polyimideresin, with the need to keep the relations of the above hardening timeand the viscosity.

Ultimately, the outlet for resist itself is sealed off by the polyimideresin or epoxy resin.

Referring finally to FIG. 10, an Sn intimate-contact layer 40 is formedbetween the external connection terminal 5 and the metal column 4 and onthe metal dome 3, the layer 40 being formed in the processing step ofFIG. 9E after the formation of a Cu thin film, by Sn vapor deposition,sputtering or the like by use of patterning of the resist B.

Although in the above embodiment a single surface acoustic wave devicehas a single surface acoustic wave element function unit by way ofexample, the application of the present invention is not limitedthereto. It would equally be possible for the surface acoustic wavedevice to have a plurality of surface acoustic wave element functionunits and have separate metal domes 3 each corresponding to each surfaceacoustic wave element function unit. It would also be possible toprovide a single common metal dome 3 for the plurality of surfaceacoustic wave element function unit.

As hereinabove set forth in the embodiment with reference to thedrawings, the present invention eliminates not only the need for thespace for wires but also the need for the cap and package itself, due tono need for the wire bonding. This enables the surface acoustic wavedevice to be created through only the piezoelectric substrate waferprocessing steps, which advantageously leads to a lowering of the costs.

In addition, implementation of the chip-size packaging on a wafer basisallows the provision of the low-priced, lightweight and small-sizedsurface acoustic wave devices.

1. A method of manufacturing a surface acoustic wave device, comprisingthe steps of: forming a drive electrode having a surface acoustic waveelement function and an external connection unit on a piezoelectricsubstrate wafer; providing a resist coat on an upper region of the driveelectrode; covering the resist coat with a metal film; providing anelectro-conductive column on the external connection unit; removing theresist coat lying within the metal film so that the metal film is formedin a dome form having a hollow portion covering the drive electrode;forming an external connection terminal at an extremity of theelectro-conductive column; and providing a resin seal on the metal film.2. The method of manufacturing a surface acoustic wave device accordingto claim 1, wherein the resin seal is provided by a first sealing stepwith a resin having high hardening speed and high viscosity and by asecond sealing step with a resin having lower hardening speed and lowerviscosity than the resin for use in the first sealing step.
 3. Themethod of manufacturing a surface acoustic wave device according toclaim 2, wherein the resin for use in the resin seal is epoxy resin. 4.The method of manufacturing a surface acoustic wave device according toclaim 2, wherein the resin for use in the resin seal is polyimide resin.